I. Field of the Invention
The present invention relates to an engine cooling system for a reciprocating piston engine.
II. Description of the Prior Art
There are many types of previously known reciprocating piston engines of the type having a cylinder with an inner and outer end. A piston is mounted within the interior of the cylinder and translates between a top dead and a bottom dead center position. In the top dead center position, the top or outer end of the piston is closely adjacent the outer end of the cylinder thus forming a relatively small combustion chamber between the top of the piston and top of the cylinder. Conversely, in its bottom dead center position, the top of the piston is spaced away from the top of the cylinder.
As is well known in the art, when the piston is at its top dead center position the piston compresses a fuel/air mixture in the combustion chamber which is subsequently ignited by any conventional ignition means. The expansion of the gases resulting from this ignition forces the piston towards its bottom dead center position. In a two-stroke engine, the fuel/air mixture is ignited each time the piston is at or adjacent its top dead center position while, conversely, in a four-stroke engine the fuel/air mixture is ignited every other time the piston is at or adjacent its top dead center position.
The ignition of the fuel/air mixture within the combustion chamber in these previously known engines creates a high heat load which is transmitted not only to the cylinder but also to the piston. This heat must be dissipated or removed from both the piston and the cylinder in order to prevent thermal damage to the cylinder and/or piston which would otherwise damage the engine.
There are two different types of previously known systems for cooling both the engine cylinder and the piston, i.e., an air cooled system and a liquid cooled system. In the air cooled system, a plurality of heat conductive fins are secured to and extend outwardly from the cylinder. These fins form a heat sink which transfers the heat from the cylinder and piston to the fins and ultimately to airflow passing through the fins.
While these previously known air-cooled engines are suitable for many applications, in many applications there is insufficient airflow past the cooling fins to obtain the desired heat dissipation. Furthermore, these previously known cooling fins are heavy and bulky in construction thus rendering them unsuitable for weight critical applications, such as aircraft engines, where the weight of the cooling system is of critical importance.
In addition to a multi-cylinder air-cooled engine is not an efficient heat transfer device as compared to a well-designed finned heat exchanger and typically requires substantially higher cooling air flow rates as compared to a radiator for an equivalent liquid cooled engine, thus representing a drag penalty for aircraft applications. Whereas, it is usually difficult to achieve a uniform distribution of cooling airflow over a multi-cylinder air-cooled engine, a liquid-cooled engine eliminates the airflow distribution problem, hence improving uniformity of cylinder-to-cylinder cooling, and further contributing to a low drag approach.
Also, typical metal temperature profiles in an air-cooled cylinder are not uniform due to the variation of the cooling airflow field around the cylinder.
As a result, combustion chamber metal temperatures may vary considerably, and the temperature profiles in the area of the cylinder barrel are uneven resulting in ovalization of the barrel during engine operation and requiring large piston to cylinder running clearances.
In the previously known liquid-cooled engines, a housing or coolant jacket encases the outer end of the cylinder and extends along the sides of the cylinder to a position below the inner end of the piston when the piston is at its top dead center position. A coolant, such as water, glycol, or the like is pumped through the cooling jacket so the heat from the cylinder and piston are transferred to the coolant and disipated elsewhere by a heat exchanger or other heat disipating means. These previously known cooling systems, while effective in operation, are relatively heavy in construction since the cooling jacket extends downwardly along the sides of the cylinder and below the inner end of the piston when the piston is at its top dead center position and often times extend along the entire length of the cylinder. On multi cylinder liquid-cooled engines, the water jacket normally encases an entire cylinder bank. As such, these previously known cooling systems are undesirable for weight critical applications, such as aircraft engines.